1. Field of the Invention
The present invention relates to semiconductor processing technology and, in particular, concerns a method of cleaning and monitoring pads used in planarizing of the surfaces of a wafer using chemical mechanical polishing.
2. Description of the Related Art
Chemical mechanical polishing or planarization (CMP) is a technique whereby surfaces, such as semiconductor substrates, are planarized by the simultaneous application of both etching and polishing processes. CMP is typically used to globally planarize surfaces such as the upper surface of a semiconductor wafer. The wafer is typically positioned within a carriage and is rotated with respect to a polishing pad. In one approach, a slurry containing abrasive particles and an etchant is interposed between the polishing pad and the surface of the semiconductor wafer that is to be planarized. The polishing pad is then brought into contact with the surface of the wafer that is to be planarized and the combination of the mechanical polishing and the etchant results in the exposed surfaces of the wafer being removed by the CMP process.
One specific technique of CMP, known as copper CMP (CuCMP), is used to remove copper (Cu) from the wafer surface. CuCMP is presently used extensively in conjunction with a copper application technique known as copper Damascene process. One method of laying metal lines and interconnects in the integrated circuits is to form a layer of metal on the wafer and chemically etch away the metal. Copper is the metal of choice over other metals such as aluminum and tungsten, due to its desirable electrical properties. Copper, however, is difficult to use in the etching technique due to its high susceptibility to corrosion during the process. Corrosion leads to unpredictable electrical properties of the resulting copper interconnects, thus making copper essentially unusable for such an application. The copper Damascene process overcomes the corrosion problem by depositing copper directly into the groove patterns of interconnects already formed within the dielectric layer of the wafer. Since the copper is not etched away chemically, copper corrosion is no longer a problem. The excess copper from the Damascene process is removed by CuCMP.
As any material is removed globally from the surface of the wafer, it is desirable to be able to stop the CMP process after a predetermined amount has been removed. Endpoint technique is a method of stopping the CMP process after a right amount of material has been removed. Typically, endpoint techniques rely on frictional properties and/or light reflecting properties of the surfaces involved in the CMP process. As a layer of material is being removed from the wafer, that layer exhibits certain friction and reflectivity. When that layer is polished off and a new layer is exposed, friction between the pad and the wafer surface changes. Also, the reflectivity of the surface changes when the new layer is exposed. The CMP system can detect either or both of these changes and establish an endpoint. One of the parameters that aids in accurate endpoint technique is the removal rate that depends on the condition of the polishing pad.
One of the problems associated with the CuCMP is that slurries used in the CuCMP process are highly reactive with copper, and the various copper byproducts end up being lodged in the pad. As the copper is removed, copper byproducts are formed and begin to clog the pores and grooves on the pad. As the pores and the grooves get clogged, the slurry cannot flow uniformly throughout the surface of the pad, and glazing may occur at various locations, thus causing a non-uniform removal rate of the pad. To overcome this problem, pads are typically cleaned prior to use on a wafer.
Pad cleaning involves restoring the surface of the pad followed by chemically rinsing away the copper byproducts from the pad. The surface is restored typically by using a diamond grinding disk that comes into contact with the pad in a manner similar to that of the silicon wafer being planarized. The abrasive diamond grinding disk breaks up any glaze that may have formed on the pad""s surface, and also restores a desired roughness of the pad""s surface. Once the surface is mechanically restored, residual particles and the copper byproducts from the pores and grooves are dissolved away using a rinse solution. A typical rinse solution comprises a 5% ammonium citrate solution.
Despite cleaning prior to each use, pads used in CuCMP still show drifts in removal rate, and recent data show that amount of copper byproducts absorbed in the pad increases over time as the pad is cycled between cleanings and uses. Some of the methods used to measure such data are disclosed in technical publications such as xe2x80x9cCu dissolution from Si(111) into an SC-1 process solutionxe2x80x9d, D. Chopra et al., Journal of Electrochemical Society, Vol. 145, No. 4, 1998, and xe2x80x9cAn optical method for monitoring metal contamination during aqueous processing of silicon wafersxe2x80x9d, D. Chopra et al., Journal of Electrochemical Society, Vol. 145, No. 5, 1998. Such measurements indicate that the present method of cleaning of pad does not remove the copper byproducts sufficiently. Furthermore, a fabricator using a typical conventional pad cleaning method does not know the actual condition of the pad.
While the current method of cleaning the pads for use in CuCMP process does remove copper byproducts, it is desirable that there be a more consistent method of cleaning and monitoring the pad. In particular, it is desirable to have a method of determining the concentration of the copper byproducts lodged in the pad accurately so that a fabricator can better understand the cleaning process so as to form an endpoint technique in the cleaning process. Additionally, it is desirable to map out the condition of the entire pad boundary so as to be able to achieve uniform cleaning that will lead to uniform removal of material from the wafer. By knowing the copper byproduct concentration over the entire boundary of a given pad, a proper cleaning and a proper endpoint technique can be worked out for that particular pad to yield a predictable and uniform removal rate, thus yielding a higher quality planarized wafer.
The aforementioned needs are satisfied by a system for cleaning a chemical mechanical polishing (CMP) pad. According to one aspect of the invention, the system comprises a CMP pad that is used to perform the CMP process on a device. The system further comprises a carriage that holds the device such that the CMP pad and the carriage are rotatable with respect to each other so as to allow the device to come in contact with the CMP pad. The system further comprises a slurry supply system that supplies slurry to the interface between the CMP pad and the device such that the combination of the slurry and the rotational movement between the CMP pad and the device results in removal of material from the device. The system further comprises a cleaning pad that rotates with respect to the CMP pad, and a cleaning solution supply system that supplies cleaning solution to the interface between the cleaning pad and the CMP pad. The combination of the cleaning solution and the rotational movement between the cleaning pad and the CMP pad results in removal of contaminants from the CMP pad. The system further comprises a cleaning solution analyzing system that analyzes the cleaning solution after the cleaning solution has been introduced to the interface between the CMP pad and the cleaning pad and determines, based upon the analysis the cleanliness of the CMP pad.
In the preferred embodiment of the invention, the material removed from the device is copper metal, and the CMP pad and the slurry are adapted to remove copper from the device. The cleaning pad is an abrasive diamond impregnated disk, and the cleaning solution comprising 5% ammonium citrate spiked with nitric acid is adapted to remove the copper oxides from the CMP pad. The cleaning solution analyzing system comprises an optical analyzing system and a chemical analyzing system. The optical analyzing system comprises a light source and a light detector. The light source directs a light into the cleaning solution towards the light detector, wherein the light detector detects changes to the light induced by the cleaning solution. One change induced by the cleaning solution is a change in the refraction of the light due to refractive index being dependent on concentration of ions from the contaminants. Another change induced by the cleaning solution is a change in the absorption of the light, wherein the absorption depends on the concentration of the contaminants. The chemical analyzing system samples the cleaning solution and also determines the concentration of the contaminants.
Another aspect of the invention comprises a system for analyzing the cleanliness of a CMP pad that is cleaned by a combination of mechanical abrasion and a cleaning solution. This analyzing system comprises a light source that projects a beam of light into the cleaning solution flow after the cleaning solution has been introduced onto the CMP pad during and after the mechanical abrasion. The analyzing system further comprises a detector that receives the light from the light source, and a controller that receives signals from the detector that are indicative of at least one characteristics of the light that is travelling through the cleaning solution flow. The controller determines the cleanliness of the CMP pad based upon the signals received from the detector.
In the preferred embodiment of the analyzing system, the beam of light is a beam of HeNe laser, and the detector is a pin-diode array that can resolve the detected beam of light spatially and by intensity. The signals from the detector comprise a change in the location and a change in the intensity of the detected beam of light, wherein the changes are induced by refraction and absorption of the light, respectively, in the cleaning solution flow in a manner described above.
In a preferred method of cleaning a CMP pad to remove contaminants, a cleaning pad is positioned adjacent the CMP pad and moved relative the CMP pad. Preferably, the cleaning pad is an abrasive diamond disk. As the cleaning pad is moved relative the CMP pad, a cleaning fluid is provided to the interface between the cleaning pad and the CMP pad so as to facilitate the cleaning. Preferably, the fluid comprises 5% ammonium citrate spiked with nitric acid. During the cleaning process, the cleaning fluid is evaluated to determine the condition of the CMP pad. Preferably, measurements of optical properties and chemical composition of the cleaning fluid yield a fluid characteristic value that is indicative of the condition of the CMP pad. Specifically, a change in the refractive index of the fluid is indicative of a change in the concentration of contaminants in the fluid. Also, a change in the absorption of light in the fluid is indicative of a change in the concentration of contaminants in the fluid. Preferably, measurement of the concentration of the contaminants in the fluid is performed chemically also.
From the foregoing, it will be appreciated that the process of the present invention allows for cleaning of the CMP pad in a manner such that the cleanliness of the CMP pad can be monitored. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.